Solid electrolytes for use in batteries and other electrochemical devices must have good ionic conductivity in addition to excellent film forming properties and good storage stability. Moreover, the solid electrolyte must be simple to produce. However, solid electrolytes which satisfy all of these requirements have not yet been developed.
For example, it is known that inorganic solid electrolytes such as Na-.beta.-A1.sub.2 O.sub.3 and Na.sub.1+x Zr.sub.2 P.sub.3-x Si.sub.x O.sub.13 (wherein x is from 0 to 3) have good ionic conductivity as described in M. S. Whittingham et al. Journal of Chemical Physics, 54, 414 (1971) and A. Clearfield et al, Solid State Ionics, 9/10, 895 (1983). However, these inorganic solid electrolytes have very low mechanical strength and are difficult to process into a flexible film.
Further it is known that complexes of polyethylene oxide and various salts of metals belonging to Group I or Group II of the Periodic Table (such as LiCF.sub.3 SO.sub.3, LiI, LiC10.sub.4, NaI, NaCF.sub.3 SO.sub.3, and KCF.sub.3 SO.sub.3) function as solid electrolytes (see P. Vashista et al, Fast Ion Transport in Solid, 131 (1979)). These complexes have good pliability and viscoelasticity, both of which are inherent to polymeric materials, and are easy to process. However, since the ionic conductivity of polyethylene oxide is highly dependent on temperature, and although exhibiting good ionic conductivity at 80.degree. C. or higher, the ionic conductivity abruptly decreases at room temperature or lower. Thus it is difficult to use polyethylene oxide in general purpose commercial products for use over a wide temperature range.
In order to overcome the abrupt decrease in ionic conductivity at room temperature or lower, as seen in such polyethylene oxide-based solid electrolytes, Japanese Pat. application No. 62-139266 proposes a method wherein a mixture of polyethylene oxide having a conventional molecular weight and low molecular weight polyethylene oxide having a molecular weight of not higher than 1,000 is used. However, this proposal does not provide a good means for solving the problems of the prior art, i.e., use of a large proportion of low molecular weight polyethylene oxide provides improved ionic conductivity at room temperature but the film forming properties are greatly impaired thus leading to difficulty in film formation.
A method for imparting good film forming properties while retaining good ionic conductivity at room temperature by chemically modifying the low molecular weight polyethylene oxide, and a method for introducing low molecular weight polyethylene oxide into the side chains of a vinyl based polymer is reported by D. J. Bannister et al, Polymer, 25, 1600 (1984). Although such a polymeric material forms a solid electrolyte in combination with LiC1O.sub.4 and has good film forming properties ionic conductivity at room temperature is not yet satisfactory.
Moreover polymeric materials comprising a combination of low molecular weight polyethylene oxide and a silicone compound ar reported by Nagaoka et al. Journal of Polymer Science, Polymer Letter Edition 22, 752 (1982), D. J. Bannister et al, Polymer Communications, 27, 648 (1988). Although these polymeric materials form a solid electrolyte in combination with, e.g., LiC10.sub.4 or LiCF.sub.3 SO.sub.3, have good film forming properties, and have good ionic conductivity at room temperature, these materials have poor storage stability in that the polymer chain is gradually severed, thus leading to a reduction in molecular weight.
Moreover, materials wherein low molecular weight polyethylene oxide is introduced into a silicone are reported by Watanabe et al, Journal of Power Source, 20, 327 (1987). However, since the rate of introduction of the low molecular weight polyethylene oxide is low, such materials cannot be satisfactorily used unless a proportionately large quantity of a polymeric material is used.
Watanabe et al discloses an ionic conductor of 10.sup.-6 Scm.sup.-1 at room temperature obtained from a network polymer of following components (A) and (B) and LiC10.sub.4 ##STR2##
Additionally, polyphosphazene having low molecular weight polyethylene oxide in the side chains thereof is reported by D. F. Shriver et al., Journal of American Chemical Society, 106, 6854 (9184). Although a solid electrolyte comprising a combination of such a polymer with, e.g., LiC1O.sub.4, exhibits good ionic conductivity at room temperature and has good film forming properties and adequate storage stability, polyphosphazene having low molecular weight polyethylene oxide in the side chains thereof has proved to be unsuitable for industrial production. The subject material is synthesized by derivation of hexachlorophosphazene into polydichlorophosphazene upon a ring opening polymerization and a subsequent reaction with a sodium salt of a low molecular weight polyethylene oxide. A first problem encountered in the production thereof resides in the polymerization reaction which converts hexachlorophosphazene to polydichlorophosphazene. As the polymerization reaction proceeds, a competing crosslinking reaction also takes place. The reactant becomes insoluble in the reaction solvent such that polymerization does not proceed satisfactorily. Accordingly, polydichlorophosphazene cannot be obtained from hexachlorophosphazene in good yield, which results in high production costs.
A second problem encountered in the production thereof resides in the reaction step between polydichlorophosphazene and the sodium salt of the low molecular weight polyethylene oxide. In general, reaction with a polymer having a functional group proceeds at a much lower reaction rate as compared to reaction with a low molecular weight compound having the same functional group. In order for the reaction to proceed satisfactorily, it is necessary to add a large excess of reactants (in the instant case, the sodium salt of low molecular weight polyethylene oxide). This results in a mixture of polyphosphazene having low molecular weight polyethylene oxide in the side chains thereof with low molecular weight polyethylene oxide, whereby the film forming properties of the derived solid electrolyte are deteriorated (as seen in Japanese Pat. application 62-139266). In order to decrease the amount of low molecular weight polyethylene oxide incorporated, the amount of the sodium salt of low molecular weight polyethylene oxide reacted with the polydichlorophosphazene may be decreased. However, as the amount of low molecular weight polyethylene oxide decreases, the rate of introduction of polyethylene oxide into the side chains of the polyphsphazene is greatly reduced. Thus, in order to attain the desired ionic conductivity, a large quantity of polymeric material must be used.
In the light of the above, the hitherto developed solid electrolytes are not capable of collectively satisfying all the necessary properties for application to batteries or other electrochemical devices.